The present invention relates to an oscillator and, more particularly, to a feedback circuit for use in the oscillator employing a piezoelectric oscillating unit.
Generally, the oscillator includes, as shown in FIG. 1, an amplifier A having a gain greater than 1 and a feedback circuit F connected between the output and input of the amplifier A. The signal produced from the output of the amplifier A is partly fedback to the input of the amplifier A for producing oscillating signal from the amplifier A.
Recently, various types of digital devices employ a clock-pulse producing circuit which uses an oscillator employing a piezoelectric oscillating unit. However, in the oscillator of the prior art, the feedback circuit includes capacitors other than the piezoelectric oscillating unit. For example, in FIG. 2, the feedback circuit F of the prior art includes a piezoelectric oscillating unit x having two electrodes 2 and 3, a capacitor C1 connected between the electrode 2 and the ground, and a capacitor C2 connected between the electrode 3 and the ground. The output of the amplifier A is connected to the electrode 3 through an input terminal 4 while the input of the amplifier A is connected to the electrode 2 through an output terminal 5. The feedback circuit F is so arranged as to cause the piezoelectric oscillating unit x to exhibit an inductive property in the frequency region between resonant frequency fr and antiresonant frequency fa. FIG. 3 shows an equivalent circuit under the operating condition of the feedback circuit F forming a Colpitts oscillator together with the amplifier A. The equivalent circuit includes resistance component Re and inductance component L which corresponds to the resistance and inductance of the piezoelectric oscillating unit produced during the oscillation of the piezoelectric oscillating unit. The resistance component Re and the inductance component L are connected in series between the input and output terminals 4 and 5. A capacitance component C1' is connected between the output terminal 5 and the ground while a capacitance component C2' is connected between the input terminal 4 and the ground. The capacitance component C1' has a capacitance equal to the sum of capacitance C1 (the reference characters used for indicating electric elements are also used for indicating their particular electrical amount) and input capacitance at the input of the amplifier A. Similarly, the capacitance component C2' has a capacitance equal to the sum of capacitance C2 and output capacitance at the output of the amplifier A.
In the above described oscillator employing a two electrode type piezoelectric oscillating unit x, it is necessary to further employ two capacitors C1 and C2.
Furthermore, the unstableness in the capacitance of the capacitors C1 and C2 causes a variation in the oscillating frequency. Therefore, in order to stabilize the oscillating frequency, it is necessary to further provide an additional electric element such as a trimmer capacitor, which results in an increased number of components and also increased manufacturing cost.